The competition on these tournaments is very soft. As for your first question, that depends on your skill, what you consider a decent profit, and how well you run. It can take hundreds or thousands of MTT's for your true win rate to become clear, due to the huge variance involved.

If you want to try them out, go for it: They are very beatable, and while I can't tell you how long it will take to make a decent profit, I can tell you that it's possible to make one.